Homo- and heterometallic oxalate-based complexes obtained using [Cr(C2O4)3]3− building block – Two polymorphs of a solvate

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Two heterometallic complex salts, as polymorphs of a solvate, consisting of three different mononuclear units, [CuCl(phen)2][Cu(H2O)(phen)2][Cr(C2O4)3]·nH2O [n = 5 (1) and n = 8 (2); phen = 1,10-phenanthroline] were obtained in reactions in which [Cr(C2O4)3]3− was used as a building unit. The polymorphs of a solvate are obtained in crystalline forms that differ in type or stoichiometry of included solvent molecules, mostly obtained under different conditions. In this [CuIICrIII] system, the used solvents and crystallization method influence the final product, i.e., the formation of a specific solvatomorph. From the reaction of an aqueous solution of K3[Cr(C2O4)3]·3H2O and acetonitrile solutions containing phen and CuCl2·2H2O, compound 1 was formed when the layering technique is used; and compound 2 can be obtained from the mixture of these reactants, but using methanol instead of acetonitrile. Based on the index parameter for the degree of trigonality, τ, the geometry of copper(II) atoms of the [Cu(H2O)(phen)2]2+ cations in 1 and 2 are distorted from square pyramidal toward trigonal bipyramidal conformation, while those of [CuCl(phen)2]+ have a trigonal bipyramidal geometry. Interestingly, in the presence of [NBu4]+ cations, and applying slow liquid diffusion, partial decomposition of the building block [Cr(C2O4)3]3− occurred and the homobinuclear oxalate-bridged complex [{Cu(bpy)Cl}2(µ-C2O4)]·CH3OH (3; bpy = 2,2′-bipyridine) was formed. Consequently, the oxalate ligand released from the coordination sphere of CrIII was coordinated to CuII ions during the crystallization process. The obtained oxalate-based compounds 1–3 were characterized by IR spectroscopy and single-crystal X-ray diffraction.